A known strategy for the use of fuel cells involves the generation of hydrogen from carbonaceous fuels. Generally, this process involves subjecting the fuel to desulfurization, steam reforming and high- and low-temperature water-gas shift reactions. The resulting gas stream comprises significant quantities of hydrogen (H2), carbon dioxide (CO2), water (H2O) and about 0.5% carbon monoxide (CO). The aforesaid quantity of CO is greater than desired for fuel cell purposes, since CO is known to poison the catalyst for the fuel cell reaction. It is therefore necessary to remove some or all of the CO, e.g., by oxidizing it to CO2, without removing the H2 needed to power the fuel cell. The CO must be removed or reduced to a maximum of about 10 ppm. In a prior art process known under the trade name SELECTOXO™, the product of the water-gas shift reactions is stripped of CO in a catalytic selective oxidation process that avoids oxidation of H2. The commercial SELECTOXO™ catalyst involved comprises from 0.3 to 0.5% platinum and 0.03% iron dispersed on alumina support tablets or pellets by wet impregnation of the alumina with a solution of platinum and iron salts. The SELECTOXO™ catalyst material was dried at not more than 125° C. because it was expected that that catalyst would be used at temperatures not higher than 125° C. and that a higher drying temperature would detrimentally affect the platinum. The catalyzed alumina tablets are typically assembled into a bed through which the feed stream is flowed.
As disclosed in commonly assigned U.S. Pat. No. 6,559,094,the entire content of which is herein incorporated by reference, superior catalytic activity for the selective oxidation of carbon monoxide can be obtained by using a catalyst comprising platinum and iron that have been impregnated onto a support material or monolith which was then dried and calcined under oxidizing conditions, e.g., in air, in the temperature range of from 200° C. to 300° C. The prior art did not recognize the advantage of the use of materials calcined in this range for the selective oxidation processes described therein. The invention as disclosed therein also relates to a method for the preparation of a catalyst and catalytic material and to the products of the method. The method comprises wetting a support material such as alumina (or a monolith of such material) with platinum and iron in solution and calcining the wetted material or monolith in oxidizing conditions, e.g., in air, at temperatures in the range of from 200° C. up to, but not including, 300° C. The loading of platinum on the support material should be in the range of from about 3 to 7 weight percent, preferably about 5 weight percent. The iron loading is roughly proportional to the platinum loading at about six percent thereof, e.g., in the range of from about 0.1 to 0.6 weight percent, preferably about 0.3 weight percent. Loadings of 3 to 5 weight percent platinum and 0.3 weight percent iron on powdered alumina correspond to the platinum and iron content in the surface layer of the prior art SELECTOXO™ catalysts described above. The overall loadings of 0.3 to 0.5 weight percent platinum and 0.03 weight percent iron stated above relative to the SELECTOXO™ catalysts reflect the fact that the SELECTOXO™ tablets contain within their interiors substantial quantities of alumina that are substantially free from catalytic species and which do not have significant contact with feed stream gases.
The literature is replete with other selective oxidation catalysts, but few are robust in a steam environment. George Avgouropoulos et al., Catalysis Letters, 73, 33, 2001,demonstrated a working preferential oxidation (PROX) catalyst based on copper and cerium, with an operating temperature above 140° C. in the presence of 10% steam. The elevated temperature is required to allow the copper to reduce and become active.
Mitsubishi Gas (U.S. Pat. No. 6,548,034) discloses the use of Pt with copper, as well as Pt with Mn, Ni or Co, for a PROX catalyst that is applied to fuel cells. A limiting factor in reducing the concentration of CO is the extent of reverse water gas shift activity of the catalyst. The normal water gas shift reaction is:H2O+CO→H2+CO2 The reverse water gas shift reaction, rWGS, for the reaction going in the opposite direction is:H2+CO2→H2O+COwhere poisonous CO gas is produced. The Mitsubishi Gas patent states that the mixture of copper and platinum does not exhibit the reverse water gas shift activity, commonly associated with Pt-containing catalysts in a gas mixture containing H2 and CO2 below 160°0 C. The data, however, in the patent is not believed to support such a conclusion.